1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method of fabricating a liquid crystal display device having a concave reflector. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for improving reflexibility of the reflector in the liquid crystal display device.
2. Discussion of the Related Art
Flat panel display devices, which have characteristics of being thin and light weight, and consuming low power, have been required as the information age rapidly evolves. The flat panel display devices may be classified into two types depending on how the light is emitted. One is a light-emitting type display device that emits the light by itself to display images, and the other is a light-receiving display device that uses an external light source to display images. A plasma display panel (PDP) device, a field emission display (FED) device, and an electroluminescent (EL) display device are examples of the light-emitting type display device, and a liquid crystal display (LCD) device is an example of the light-receiving type display device. The liquid crystal display device is widely used for notebook computers and desktop monitors, etc. because of its superior resolution, color image display, and quality of displayed images.
Generally, the liquid crystal display device has first and second substrates, which are spaced apart and face into each other. Each of the substrates includes an electrode, and the electrodes of each substrate are positioned to face into each other. A liquid crystal layer is interposed between the first substrate and the second substrate. A voltage is applied to the electrodes of each substrate, and thus the alignment of the liquid crystal molecules is changed according to the voltage applied to display images. Because the liquid crystal display device cannot emit the light as described above, it needs an additional light source to display images.
Therefore, a color image may be displayed by irradiating artificial light from a backlight, which is positioned behind the liquid crystal panel, to the liquid crystal layer and then controlling the amount of the light according to the alignment of the liquid crystal. At this time, the electrodes are formed of a transparent conductive material and the substrates are transparent. This liquid crystal display (LCD) device is commonly referred to as a transmissive LCD device. Because the transmissive LCD device uses an external light source such as a backlight, it can display a bright image in the dark surroundings but it has a high power consumption.
To solve this problem, a reflective LCD device has been proposed and developed. The reflective LCD device displays a color image by controlling a transmittance of the light according to the alignment of the liquid crystal molecules by reflecting ambient light or external light. The reflective LCD device depends on ambient light or an external light source for its light source and accordingly it has a lower power consumption than the transmissive liquid crystal display device. In the reflective LCD device, the lower electrode may be formed of a material that reflects light well, and the upper electrode may be formed of a transparent conductive material.
FIG. 1 is a schematic cross-sectional view of a reflective liquid crystal display device according to the related art.
In FIG. 1, first and second substrates 11 and 21 are spaced apart from each other. A gate electrode 12 is formed on the inner surface of the first substrate 11, and a gate insulating layer 13 is formed on the gate electrode 12. A gate line (not shown), which is connected to the gate electrode 12, is also formed between the first substrate 11 and the gate insulating layer 13. An active layer 14 and an ohmic contact layer 15a and 15b are subsequently formed on the gate insulating layer 13 over the gate electrode 12. Source and drain electrodes 16b and 16c are formed on the ohmic contact layer 15a and 15b, and the source and drain electrodes 16b and 16c form a thin film transistor having the gate electrode 12. A data line 4, which may be formed of the same material as the source and drain electrodes 16b and 16c, is formed on the gate insulating layer 13. The data line 4 is connected to the source electrode 16b and crosses the gate line, thereby defining a pixel area.
A passivation layer 17, which may be formed of an organic material, is formed on the source and drain electrodes 16b and 16c. The passivation layer 17 covers the thin film transistor and has a contact hole 17a exposing a portion of the drain electrode 16c. A reflective electrode 18, which may function as a pixel electrode, is formed on the passivation layer 17 at the pixel area. The reflective electrode 18 is connected to the drain electrode 16c through the contact hole 17a. Here, the reflective electrode 18 is formed of a conductive material such as a metallic material and has an undulating surface for diffusing light in order to increase reflectance at effective viewing angles for a user. The reflective electrode 18 covers the thin film transistor and overlaps the data line 4 to increase an aperture ratio of the device. At this time, the passivation layer 17 may be formed of an organic material having a relatively low dielectric constant so as to prevent signals of the reflective electrode 18 and the data line 4 from interfering each other.
Meanwhile, a black matrix 22 is formed on the inner surface of the second substrate 21, and a color filter layer 23a, 23b, and 23c having three colors of red (R), green (G), and blue (B) is formed on the black matrix 22. A common electrode 24 of a transparent conductive material is formed on the color filter layer 23a, 23b, and 23c. Each color of the color filter layer 23a, 23b, and 23c corresponds to each reflective electrode 18, and the black matrix 22 is disposed to the corresponding edges of the reflective electrode 18. As stated above, since the reflective electrode 18 of an opaque conductive material such as metal covers the thin film transistor, the black matrix 22 can cover only the edges of the reflective electrode 18.
A liquid crystal layer 30 is interposed between the reflective electrode 18 and the common electrode 24. When a voltage is applied to the reflective electrode 18 and the common electrode 24, liquid crystal molecules of the liquid crystal layer 30 are arranged by an electric field induced between the reflective electrode 18 and the common electrode 24. Although not shown in the drawing, alignment layers are formed on the reflective electrode 18 and the common electrode 24, respectively, to arrange the liquid crystal molecules of the liquid crystal material layer 30.
In the reflective liquid crystal display device, images are displayed by reflecting light from the outside at the reflective electrode, which is formed with a material that reflects light well. Accordingly, the reflective liquid crystal display device may be used for a long period of time with a limited power due to a low power consumption.
In addition, since the passivation layer 17 has an undulated surface, the reflective electrode 18 also has an undulated surface. The undulated surface changes a reflected angle of light to increase brightness at the front side.
A manufacturing process of an array substrate for a reflective liquid crystal display device according to the related art will be described with reference to the accompanying drawings. Here, since the process steps up to the source and drain electrodes are equal to the manufacturing process of a conventional liquid crystal display device, detailed descriptions are omitted for simplicity.
FIGS. 2A to 2F are cross-sectional views showing the manufacturing process of an array substrate for a reflective liquid crystal display device according to the related art.
In FIG. 2A, a thin film transistor and a passivation layer 17 are formed on a substrate 11, and a first organic layer 31 is formed on the passivation layer 17 by coating an organic material. The organic material may be a photosensitive material, such as photo-acryl. The photosensitive material does not require an additional photoresist for patterning.
In FIG. 2B, the first organic layer 31 of FIG. 2A is exposed to light with a mask and is then developed to form organic patterns 33 having a definite size and a regular interval therebetween. At this time, a slanting angle of an uneven surface to be formed in a later process may be controlled by adjusting a space or overlap between the organic patterns 33. A portion of the organic material exposed to light may be removed. Alternatively, an exposed portion may be removed.
In FIG. 2C, the organic patterns 33 of FIG. 2B are annealed to form undulated organic patterns 35. At this time, the undulated organic patterns 35 are melted by annealing and are spread on passivation layer 17. Next, the undulated organic patterns 35 are hardened so as to have a slanting angle and to be slowly curved.
In FIG. 2D, a second organic layer 37 is formed on the entire surface of the passivation layer 17 including the undulated organic patterns 35 thereon by coating an organic material. Here, the side surface of the second organic layer 37 may be adjusted due to the undulated organic patterns 35, and the surface of the second organic layer 37 may be curved having an inclined angle.
In FIG. 2E, a drain contact hole 17a is formed by patterning the passivation layer 17 and the second organic layer 37 using a mask. The drain contact hole 17a exposes a portion of a drain electrode 16c. 
In FIG. 2F, a reflector 18 is formed on the second organic layer 37 in a pixel area using a mask. The reflector 18 has an undulated surface due to the undulated organic patterns 35 and functions as a reflective electrode that reflects light and drives liquid crystal molecules.
In the convex reflector formed by the above-mentioned method, the uneven surface is formed of photo-acryl, which is a photosensitive resin. However, the photo-acryl is not used in the transmissive liquid crystal display device, and thus, the uneven surface of the reflective liquid crystal display device cannot be formed using the existing equipment. More specifically, since the photo-acryl uses a developer different from other organic materials, an investment in equipment for new developing line is required.